Internet Protocol (IP) telephone systems such as Skype have become commonplace in recent years. The IP telephone system can be used simply by installing software on a network terminal or the like, where the software is developed by the IP telephone provider for originating and receiving calls and implementing voice communications. Skype, which offers a free IP telephone application and free calls, has particularly enjoyed much popularity. One factor enabling Skype to offer free calls may be its peer-to-peer communications, which eliminate the need for a server.
United States Patent Application Publication Nos. US2005/141529A1 (Document 1) and US2007/189266A1 (Document 2) also disclose the IP telephone systems. Document 1 merely describes a communication system which can execute a function not supported by a communication apparatus itself but possessed by another device. Document 2 describes that a transmitting device transmitting images and a receiving device receiving the images can identify which communication method is supported by both parties using SDP media stream descriptions expanded by an SIP message.